1. Field of the Invention
Systems have heretofore been developed for reproducing signals at video frequencies from information recorded on discs, tapes, or other media. Such systems have utilized among other things, optical recordings upon photosensitive discs, electron beam recording on thermo plastic surfaces and, in prior patents assigned to the assignee of the present invention, systems utilizing a rotating disc which is responsive to impinging radiation to reflect or transmit radiation corresponding to and representative of the information stored on the surface of the disc.
For example, in U.S. Pat. No. 3,530,258, issued to David Paul Gregg and Keith O. Johnson on Sept. 22, 1970, there was shown and described a system in which a video signal transducer included a servo controlled pair of flexible, fibre optic elements. An air bearing supported an objective lens system. A light source of radiant energy was positioned below the disc and the transducer was responsive to transmitted light.
Other patents have shown the use of a radiant source which directed an energy beam to the surface of the disc and provided a transducer that was responsive to reflected energy. One of the major problems to be encountered in the recording and reproduction of video information, arises directly from a consideration of the energy levels involved in such a process and the restraints imposed by the considerations of size, weight and operating conditions.
To be commercially desirable as a home instrument, the system should be able to store and reproduce a "program" of at least 15 to 30 minutes in length. The record disc should be of an easily handled size, comparable to the phonograph records currently in use. If the playback turntable was operated at 1800 rpm, some 54,000 revolutions would provide 30 minutes of playback. Assuming a 1 micron track width and 1 micron spacing between adjacent racks, a circular band approximately, 4.25 inches wide is required. Assuming that the smallest radius at which information can be stored is approximately three inches, the resultant disc is about 15 inches in diameter. The duration of the program or the speed of the turntable can change the dimensions of the recorded area, as can the width of the individual track and the spacing between tracks.
Assuming that the video information has been recorded in some digital fashion, the presence or absence of a signal can be detected at an appropriate information rate. If the width of the track is approximately one micron, and the space between adjacent tracks is also one micron, the quantity of energy necessary to impart information from the disc can be determined. It is necessary to provide sufficient radiant energy to "illuminate" a "spot" of approximately one micron in diameter and, at the same time, provide sufficient radiant energy at the detector, so that the "presence" or "absence" of a signal can be distinguished.
It has been discovered, in attempting to utilize the transmitted radiation techniques of the prior art, that the provision of an inordinately large amount of radiation into the system is required in order to "transmit" a sufficiently useful increment of energy for detection through the record. It has also been determined that a substantial magnification is required to enable a state-of-the-art transducer to respond to a one micron diameter radiant spot.
If a light source illuminates the entire field which can be scanned by the detector under control of the servo system, it will be seen that an extraordinary light intensity must be provided before the light transmitted through or reflected from the disc will be of sufficient intensity to register upon the photosensitive device.
In a preferred embodiment of the present invention, an articulated mirror is utilized with a highly collimated source, to move the point of impingement of the radiant spot upon the disc. An important aspect of the present invention is the ability to direct or "steer" the illuminating radiation to a particular spot and to return the information from the spot thus illuminated to a detector system.
An important aspect of the present invention is the ability to direct the illuminating radiation to a particular spot and to return the information from the spot thus illuminated to a detector system. The prior art has suggested the use of a pair of transducers in conjunction with a summing amplifier to provide signal information and a differential amplifier to provide feedback servo information for error correction. However, given the limitations of the extremely low radiation levels, the diffraction limited characteristics of the image, and the extreme sensitivity of the system to noise and vibration, such an approach is not entirely satisfactory. A difference "curve following" technique described in the patent to W. D. Munro, U.S. Pat. No. 2,838,683, issued June 10, 1958, has suggested an alternative solution.
In the preferred embodiment therefore, a single photosensitive pickup is used as one input to a differential amplifier, and a second input is provided from a fixed bias source. The bias is adjusted to balance the input of the photodetector when it is illuminated by the reflected spot that is approximately half way into the information track, for example, on the periphery side of the track. If the intensity of the radiation upon the detector increases in a system where the track is "darker" than the band between "tracks", then a servo signal is developed to drive the mirror in a first direction, tending to move the spot toward the track and toward the center. Similarly, if the radiation decreases, the relatively higher magnitude of the bias causes an error signal to be generated which moves the mirror and the "spot" in a respectively opposite direction, away from the track and toward the periphery.
Since, in the preferred embodiment, one revolution of the disc represents one "frame" of the T.V. picture, an error in tracking, where the track is "lost", merely results in either the skipping or the repeating of a frame, both of which are undetectable by the human observer. In alternative embodiments, it is possible to use the earlier prior art technique of the photodetector pair.
A second, articulated mirror may be provided which rotates in a second direction, orthogonal to the direction used for the radial steering of the beam and tracking of the image. Such steering may be considerred to be in the circumferential direction and would aid in the synchronization and timing of the recorded information with respect to the timing frequencies generated in the reproducer circuits. As is known, television circuits, and especially color television circuits, require extremely accurate time synchronization in order to maintain color fidelity. Therefore, any error in synchronism between the local oscillator of the reproduction apparatus and the timing information recorded on the disc, may be resolved and eliminated through the use of mirror motion in the second direction.
It has been found that any errors resulting from eccentricity of the disc can be simply corrected. It will be noted that the steering and tracking circuit which maintains the radiant spot on the appropriate spiral track will undergo some periodic signal fluctuation that is related to eccentricity. It can then be shown that the change in instantaneous velocity in the circumferential direction also changes in substantially similar fashion, but lags by one-quarter revolution of the disc. Therefore, it is possible either to sense the velocity changes from the recorded timing information and from this derive a correcting signal to drive the tracking servos, or to sense the eccentricity from the tracking servo and use that signal with an appropriate phase shift to drive the "timing" servo to correct for velocity changes due to eccentricity. In an alternative embodiment, a single axis articulated mirror corrects for tracking and electronic circuits compensate for timing errors.
Accordingly, it is an object of the present invention to provide an improved playback assembly for a disc upon which video information has been recorded.
It is yet another object of the invention to provide an improved tracking circuit for optically scanning a video disc.
It is yet a different object of the invention to provide a video disc playback assembly which directs a radiant spot to the surface of the disc and directs the returning radiation to a photosensitive detector, which detects returning radiation from the disc surface.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings in which several preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.